The present invention relates to radio tracking systems for locating a mobile radio transmitter and for determining if the mobile radio transmitter has moved outside of a set range measured from a radio receiver and to mobile radio transmitters which transmit an alarm of a user of the radio transmitter to a radio receiver, and methods of operation thereof.
Parents are becoming increasingly concerned that their children may be harmed when they are out of their sight. Almost daily the media reports events involving small children being harmed when the small children have wandered from sight of their parents. Furthermore, in today""s increasingly mobile society families with small children regularly visit malls, amusement parks and other public places where crowds of people are found which provide an environment where small children can be harmed or become lost or wander from sight of their parents because of their natural inquisitiveness, tendency to explore their surroundings, or their desires to be free from control of their movements by their parents.
Devices are commercially available to limit or monitor movements of children. Devices exist for tethering children to their parents. Further radio systems are commercially available which generate an alarm when children move outside a radius from a radio receiver which receives transmissions from a transmitter worn by children. The tethering devices have a limited restraint radius and create animosity between a child and the parents. The radio systems have a fixed radius of approximately fifteen feet which is too small to permit useful monitoring if a parent does not wish to totally keep a child in sight and cannot be used for tracking.
Numerous radio tracking systems have been proposed which utilize radio communications to locate a mobile radio transmitter and/or to determine when a mobile radio transmitter carried by a person has exceeded a set range measured from a radio receiver. These systems have one or more radio transmitters which broadcast a coded identification of each radio transmitter which is received by a radio receiver and processed to determine the distance and, in some of these systems, the direction between each transmitter and receiver. See U.S. Pat. Nos. 4,785,291, 5115,223, 5,119,072, 5,245,314, 5,289,163, 5,307,053 and 5,357,259, Patent Application WO 87/06748, U.K. Patent Application GB 2182183A and Japanese Patent Application No. 64-311842. A wide range of implementations of radio tracking systems are described in the above-referenced patents and published applications.
The determination if a mobile radio transmitter has moved out of range from a radio receiver receiving an identification code of the radio transmitter is accomplished in many different ways in these patents and applications. Two ways which are described for determining if a mobile transmitter has moved out of range are by determining if the received identification code signal has dropped below a predetermined signal strength or the received identification code signal has not been received for an elapsed time interval.
Radio communication systems which are designed to determine when a mobile transmitter worn by a person has moved outside of a set range and/or to track a person encounter severe problems because of (1) limitations of transmitter power imposed by the Federal Communications Commission which limit broadcast power below 100 milliwatts, and (2) various environmental factors which cause interference, fading, or signal attenuation of the identification code signal which is periodically sent from the mobile radio transmitter to the monitoring radio receiver. The transmitter identification code signal may be severely attenuated by passage through the bodies or body parts of people or other structures in the line of site between the radio transmitter and the radio receiver. The presence of people and structures in the line of sight causes substantial attenuation of the transmitted identification code signal which may cause the identification code of the radio transmitter to be periodically or permanently attenuated below the discrimination level of the radio receiver causing a false indication that the mobile radio transmitter has moved out of a set range and an inability to further track the mobile radio transmitter.
Furthermore, natural fading phenomena, such as Rayleigh fading, which is a function of the transmitting frequency and the relative velocity between the mobile radio transmitter and radio receiver are severely aggravated by low speed movement, such as when a child or patient is walking with a transmitter attached to their person to facilitate their tracking. These fading phenomena affect the determination if a set range has been exceeded and a direction determination of the transmitter relative to the receiver. Additionally, other man-made interferences, such as electrical noise and multipath interference caused by buildings, can periodically cause the identification code signal transmitted from the radio transmitter to be attenuated to a level below the discrimination level of the radio receiver tracking the transmitter which also causes a false indication that the radio transmitter is outside a set range and/or the inability to track the direction of the radio transmitter movement relative to the radio receiver with a directional antenna.
Error correction code may be transmitted in a frame of bits encoding the identification code of the radio transmitter. One or more frames encoding the identification code of the transmitter may each contain a set number of error correction code bits which are processed by the radio receiver to correct minor bit errors such as one or two bits which occur within the identification code frame bits. One well known error correction code for accomplishing this function is the BCH code.
The serial processing of the bits of frames which contain error correction code is typically implemented with a series of EXCLUSIVE OR gates. When a number of bit errors in a frame exceeds the error correction capacity of error correction code, the data within the frame is erroneous. The prior art methods of wireless data transmission do not permit the recovery of valid data bits from a frame containing a number of bit errors which exceed the bit error correction capacity of the error code therein which error correction capacity, for most types of error correction codes, is two bits.
The cumulative effects of mis-synchronization of a radio receiver to receive transmissions from radio transmitters, Rayleigh fading, and man-made noise noticeably reduces the reliability of current digital radio receivers to receive error free data. A gap in a data transmission in excess of 1 millisecond may cause a radio receiver to terminate the receiving process. In a situation of tracking a radio transmitter with a radio receiver which receives a periodic digital transmission of the radio transmitter""s identification code, termination of the receiving process results in the correct identification of the radio transmitter not being received. As a result, the transmission from a radio transmitter which is, in fact, within a set range of a radio receiver which is monitoring the distance of the radio transmitter from the radio receiver is falsely received as being out of range. This results in an erroneous condition of monitoring the distance of the radio transmitter from the radio receiver and further, may cause a panic situation or otherwise cause the person using the radio receiver to not trust the reliability of the radio tracking system.
An analysis of wireless prior art data transmission protocols in accordance with accepted mathematical relationships for their evaluation reveals that they are poorly suited for data transmissions of more than a few characters in length. The following mathematical relationships are used to analyze fading:
Fo=SF/670xe2x80x83xe2x80x83(1)
S=Speed MPH
F=Frequency in MHz
Fo=Hz
t=xc2xd rFo (e+0.693 r2xe2x88x921)xe2x80x83xe2x80x83(2)
r=ST/SM Threshold/Median
The threshold ST is the receiver threshold detection level and the median SM is the median field strength level.
FR=2rexe2x88x920.693 r2Foxe2x80x83xe2x80x83(3)
P(error)=1xe2x88x92e FRLPwxe2x80x83xe2x80x83(4)
L=Message Time (Length)
Pw=Probability of fade larger than catastrophic failure length
Pw=1.5exe2x88x921.1 t/{overscore (t)}
The quantity {overscore (t)} is the net probability of a fade divided by the mean rate of fading and equals
xc2xd rFo (e+0.693 r2xe2x88x921)xe2x80x83xe2x80x83(5)
The fading rate Fo is the natural frequency at which atmospheric radio frequency transmissions periodically fade as a function of the channel frequency Fo and the speed of the radio receiver in miles per hour; the fade length t in seconds is the length of fade; the fade below threshold FR is the time duration in seconds that a transmission drops below the detection capability of the radio receiver; and the probability of message loss P(error) is the probability that a message transmission will not be completed as a result of a loss of synchronism between the data transmission and the receiver. See S. 0. Rice; Statistical Properties of a Sine Wave Plus Random Noise; Bell System Technical Journal, January, 1948; T. A. Freeburg; An Accurate Simulation of Multipath Fading; Paper; 1980; Caples, Massad, Minor; UHF Channel Simulator for Digital Mobile Radio; IEEE VT-29; May 1980; and P. Mabey, D. Ball; Application of CCIR Radio Paging Code No. 1; 35th IEEE V.T. Conf.; May 1985 for a discussion of the above-referenced equations.
U.S. Pat. No. 4,868,885 discloses the rapid measurement of a received signal strength indicator (RSSI signal) generated from reception of a received radio frequency signal which is used in a cellular radio system to control handoff. Samples of the RSSI signal are taken successively in time and compared with the larger of the two samples being stored throughout a desired sampling interval. Sample values exceeding the value obtained from an immediately preceding sample time and a value obtained from an immediately succeeding sample time are stored twice while samples values that are less than an immediately preceding or succeeding sample value are never stored. The resulting average is very close to a true average signal amplitude and is unaffected by Rayleigh fading phenomena but is responsive to rapid changes in received signal amplitude caused by obstacles in the transmission path.
U.S. Pat. No. 5,193,216 detects when a radio receiver of the type which receives data transmissions is out of range. The radio receiver responds to a decreasing slope of a RSSI signal after the receiver fails to receive its coded identification code from the transmitter to signal the out of range condition. The ""216 Patent discloses sampling the received signal strength coincident with the detection of a predetermined characteristic of the signal, such as the sync code, so that the signal for which the received signal strength is measured is indeed the desired signal. If at the time the sync code is to be detected there is no signal which is detected, a predetermined number of the most recently stored RSSI values are read. If the slope of the stored RSSI values indicates that the radio receiver was moving toward an out of range condition before the loss of reception, a display is generated upon loss of reception indicating that the radio receiver is out of range from the radio transmitter.
Loop antennas and their characteristics are well known. Loop antennas were originally used as directional antennas for direction finding applications. However, loop antennas are now also widely used in miniature radio receivers, such as pagers, as omnidirectional antennas.
To achieve a maximum omnidirectional response with a loop antenna, several design constraints must be observed. Typically, the length of the loop antenna must be less than approximately 0.4 wavelength of the frequency band being received with 0.2 wavelength being optimal; a high ratio of length to diameter of the loop electrical conductor (typically 100:1) is desirable and the loop should be formed into a perfect square. When operated in the aforementioned fractional wavelength configuration, the loop antenna receiving characteristics typically are dependent upon the cross-sectional area of the perfect square of the antenna configuration. This configuration, however, has a low feed resistance (less than 5 ohms) and has a relatively high inductive reactance which must be cancelled with series capacitors inserted into the loop. In high frequency bands, such as 900 MHz., even a single turn loop of a loop antenna may have 200-300 ohms of inductive reactance.
Directional beam antennas are known which use multiple active and passive elements in a configuration typically having a quarter wavelength between the elements to provide directivity and gain in a plane of the elements. This configuration is not applicable to miniaturized radio products in high frequency bands, such as 900 MHz.
Phased antenna arrays may be used for direction finding applications. Phase arrays typically rely upon tuning networks that change the phase relationship between the receiving elements so as to exhibit, directional characteristics. However, phased arrays are, also not applicable to miniaturized products in high frequency bands, such as 900 MHz.
None of the aforementioned directional antenna configurations are applicable to integration into a miniaturized radio tracking receiver of a small form factor which is ergonomically acceptable to be worn or carried by a person to track mobile objects, such as pets or children.
The present invention is an improved radio tracking system comprised of a mobile radio frequency receiver and at least one mobile radio frequency transmitter. Each radio frequency transmitter periodically broadcasts a radio frequency carrier which is modulated with an identification code which uniquely identifies the broadcasting radio frequency transmitter which is decoded, by the radio frequency receiver. The radio frequency receiver has an adjustable range control which sets a, maximum range of movement of each radio frequency transmitter measured from the xe2x80x2radio frequency receiver that is permissible without the generation of an alert that a radio frequency transmitter has exceeded the set, range. The range setting generates a voltage having a numerical value which is compared to a RSSI signal to determine if the set range has ben exceeded. When the radio frequency receiver verifies that an identification code transmitted with a modulated radio frequency carrier is assigned to a radio frequency transmitter which is being tracked or monitored by the radio frequency receiver, the radio frequency receiver generates the RSSI signal which is processed by a processor within the radio frequency receiver to compute an average of successively received RSSI signals from each of the radio frequency transmitters being monitored. The average is compared to the numerical value representing the set range by the processor and the processor alerts the user of the radio frequency receiver when the set range for any receiver is exceeded.
Preferably, each RSSI signal is integrated to remove the effects of electrical noise before averaging. The average of RSSI signals and preferably the average of the integrated RSSI signals generated from transmissions of the radio frequency carriers containing the identification code of each radio frequency transmitter being monitored and tracked are compared to the numerical value representing the set range and an alert is generated by the microprocessor (preferably a digital signal processor) of radio frequency receiver when the comparison reveals that at least one of the at least one radio frequency transmitter is outside the set range.
Preferably, the average of the RSSI signals and the preferred average of the integrated RSSI signals is updated to include newly calculated RSSI signals and preferably, newly calculated integrals of the RSSI signals only when each newly calculated RSSI signal or integral thereof differs from the computed average by less than a function of the average so as to exclude from the computation of the average those RSSI signals or integrals thereof which differ from the average by more than the function. This process discards unreliable and statistically aberrant RSSI signals or integrals thereof which unreliable and statistically aberrant RSSI signals or integrals thereof would interject erroneous data into the range determination process. Phenomenon, such as interference from people in the line of sight, Rayleigh fading, multipath interference, etc., can cause substantial magnitude variation of the magnitude of successively received RSSI signals or integrals thereof which falsely would be interpreted as motion of a radio frequency transmitter outside the set range which is not occurring and which would cause an erroneous alert to be generated that a radio frequency receiver has moved outside the range.
Once the radio frequency receiver determines that a radio frequency transmitter has moved outside the set range, the user may switch the antenna configuration from an omnidirectional antenna to a directional antenna by closing a xe2x80x9cfind mexe2x80x9d switch in the housing of the radio frequency receiver to permit directional tracking by the radio frequency receiver. Also, directional tracking may be performed by closing the xe2x80x9cfind mexe2x80x9d switch any time the user of the radio frequency receiver desires to monitor the position or motion of each radio frequency transmitter being monitored.
A display of the magnitude of successive RSSI signals and preferably, integrals thereof, which are generated in response to the radio frequency receiver detecting the radio frequency carrier containing the identification code of the radio frequency receiver being tracked, is used to locate a direction from which a maximum signal magnitude of the signal radio frequency carrier is being transmitted by the radio frequency transmitter being tracked. The direction from which the maximum magnitude signal is being received, which is detected by displaying the magnitude of a quantity which is a function of individual RSSI signals generated by the reception of sequential transmissions of the identification code of the radio frequency transmitter being tracked is the true bearing of the radio frequency transmitter relative to the radio frequency receiver. A preferred function without limitation is the integral or average signal magnitude of the RSSI signal which has the effects of noise removed.
The present invention further permits a user of each radio frequency transmitter to close a xe2x80x9cpanicxe2x80x9d switch to generate an alert which the user of the radio frequency receiver responds to by closing the xe2x80x9cfind mexe2x80x9d switch to cause the control processor to change the antenna configuration of the radio frequency receiver from an omnidirectional antenna used for tracking all of the radio frequency receivers to a directional antenna to permit directional tracking of the user of the radio frequency transmitter which transmitted the alert to the radio frequency receiver. The directional tracking process by the radio frequency receiver of each radio frequency transmitter transmitting an alert is the same as the tracking function described above when a radio frequency transmitter exceeds the set range.
The processor of the radio frequency receiver further utilizes error correction code which is transmitted with the frames of information encoding the identification code of each radio frequency transmitter which is being monitored or tracked to reconstruct valid data from frames which cannot be corrected using the error correction code. In a preferred embodiment of the invention, an IDENTIFICATION FRAME GROUP, which is comprised of a plurality of frames with each frame containing bits of BCH error correction code and bits of many of the frames encoding the identification code of the radio frequency transmitter and one of the frame encoding the status of the user of the radio frequency transmitter, is processed by the radio frequency receiver to determine if at least one erroneous uncorrectable bit is contained in any of the frames. Those frames containing at least one erroneous uncorrectable bit, which cannot be corrected by processing with the error correction code, are further processed to reconstruct valid data in the frame containing the at least one erroneous uncorrectable bit by searching for a bit pattern of the erroneous uncorrectable bits being totally within the bits of the error correction code bit field. When the bits of the error correction code of a frame totally contain the erroneous uncorrectable bits within the frame, the data which is the identification code, status of the user of the radio frequency transmitter or any other information may be recovered. The bit pattern is a number of successive bits having an identical numerical value of either zero or one with the number being at least one greater than a number of bits which may be corrected with the error correction code in the frame which contains the at least one erroneous uncorrectable bit. As a result of reconstruction of frames by recovering valid data from frames containing at least one erroneous uncorrectable bit, a greater number of radio frequency carriers containing the identification code of the radio frequency transmitters being monitored are-detected. This enables the processing of a greater number of RSSI signals which enhances the data which is processed to determine the range and direction of the radio frequency transmitters being monitored as described above.
In a preferred embodiment of the invention, the identification code of each of the radio frequency transmitters being monitored is encoded in frames containing error correction code. The bits of the frames modulate a subcarrier and the subcarrier modulating the radio frequency carrier. Analog modulation of the subcarrier or digital modulation of the subcarrier may be used. The analog modulation modulates cycles of the subcarrier with bits encoding the plurality of frames of the identification code and any other information such as the information in the IDENTIFICATION FRAME GROUP. Each cycle of the analog subcarrier is modulated by bits at a plurality of separated angular positions. Digital modulation of the subcarrier modulates a pulse width of the subcarrier. The width of parts of the digital subcarrier are modulated with at least one bit of the frames of the information. This form of subcarrier modulation permits the preferred form of data transmission as formatted into the IDENTIFICATION FRAME GROUP to be rapidly transmitted at a low error rate which enhances battery life.
The processing of the detected individual cycles of the subcarrier by the digital signal processor of the radio frequency receiver includes calculating an integral of at least one selected modulated part of each of the individual cycles, numerically comparing each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral and substituting for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit when the subcarrier is an analog subcarrier and at least one bit when the subcarrier is a digital subcarrier. Furthermore, the processing of the detected individual cycles of the subcarrier by the digital signal processor includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced. The compared sample value is preferably replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which exceeds the compared sample value.
The above-described processes, which are performed by a digital signal processor of the radio frequency receiver for processing the modulated cycles of the subcarrier, ensure that reliable detection of the identification code of each radio frequency transmitter is achieved and reliable data which is a function of the RSSI signal generated during the reception of a valid identification code of one of the radio frequency, transmitter being monitored is used to determine the range and direction of a radio frequency transmitter relative to the radio frequency receiver. The reliability of the range detecting function and further the tracking function of each radio frequency transmitter upon the generation of an alert by the radio frequency receiver when a radio frequency transmitter moves out of range or further when a user of the radio frequency transmitter pushes the panic switch is directly influenced by the reliability of the detection process of the identification code of the radio frequency transmitter. The RSSI signals, which are used ultimately to determine if a radio frequency transmitter has moved outside the set range and further to track the direction of a radio frequency transmitter relative to the radio frequency receiver, are qualified by an accurate and high speed detection of the identification code of each radio frequency carrier which is transmitted from each of the radio frequency transmitters being monitored. Therefore, a highly accurate detection process of the identification code of each radio frequency transmitter by the radio frequency receiver insures that the maximum number of qualified RSSI signals are presented for further processing which enhances the accuracy of the determination if the range set by the user of the radio frequency receiver has been exceeded and further, the accuracy of the detection of the direction of the radio frequency transmitter relative to the radio frequency receiver.
Furthermore, in accordance with the invention, the housing containing the receiver has a display to permit the user of the receiver, who is directionally tracking at least one transmitter transmitting radio transmissions to the receiver, to view the strength of the received radio transmissions to facilitate radio tracking. A field of view limiter is associated with the display and the housing to limit a field of view of the display of the strength of the radio transmissions to within a field of view causing the user of the receiver to hold the receiver at the waist or above and away from the body of the user to minimize radio interference with the: transmissions in the line of sight between the at least one radio transmitter and the receiver. Preferably, the field of view limiter causes the user of the radio receiver to hold the receiver away from the body and at or above chest level. The field of view defined by a pair of straight lines representing light rays; respectively extends from opposed edges of the display to corresponding opposed edges of an opening within the housing. The opening extends inward from an outer surface of the housing to define a recess having a bottom within the housing. The display is mounted on the bottom. The field of view subtends an angle which preferrably is no greater than 45xc2x0 and preferably 30xc2x0 or less.
Moreover, a switch for activating the directional antenna of the receiver is positioned relative to the housing so that the hand of the person using the receiver unit to directionally track the at least one transmitter holds the switch in a closed position with the directional antenna being positioned relative to the housing so that during the holding of the with in the closed position a line of sight between the antenna of the receiver and the at least one radio transmitter is not occluded by the hand of the person holding the switch in the second position.
The aforementioned field of view limiter causes the user of the receiver unit to position it relative to the user""s body to provide optimal radio reception of low power transmissions from the at least one transmitter being monitored. Reception of low power transmissions is important with the present invention because of its preferred use of small batterys to provide electrical power over many hours of continued use (e.g. 40 hours or more). In this circumstance, the radiated power from the at least one transmitter may be as low as five milliwatts which makes minimizing all forms of interference and positioning of the receiver in an optimal position to provide maximum received signal strength extremely important to achieve maximum distance of reception between the at least one transmitter and the at least one receiver and maximum directional sensitivity.
The positioning of the receiver in a position at or above the waist away from the body of the user provides a spacing of one or more wavelengths of the carrier of the transmissions which minimizes body interference and maximizes the height of the antenna of the receiver which also enhances signal reception. Moreover, positioning of the switch which activates the directional antenna relative to the housing of the receiver which requires the hand of the user to close the switch while the hand is positioned out of the line of sight between the. antennas of the at least one transmitter and the receiver also minimizes interference caused by the user""s hand.
Furthermore, the use of frequency hopping spread spectrum transmissions by the receiver and the at least one transmitter permits acceptable and sufficiently accurate matching of identification code digits to qualify the received signal strength indicator signal for further signal processing as described below without a complete match of stored and received identification code digits to achieve a reliable decoding of the identification code. Once a frequency hopping radio frequency receiver is synchronized to hop synchronously with the at least one frequency hopping radio frequency transmitter being monitored for range and/or direction, a partial identification digit match between the transmitted identification code digits and the receiver""s stored complete transmitter identification code digits, which the synchronized frequency hopping radio frequency receiver is assigned to monitor, provides statistically reliable decoding sufficient to qualify the corresponding received signal strength indicator signal for further processing which contributes to the generation of a, highly reliable processed signal as described below used for range and/or directional tracking. It is statistically improbably that a receiver will partially decode the identification code digits of a transmitter which is not synchronously frequency hopping with the receiver.
A preferred antenna design for implementing the combined omnidirectional and directional antenna of the radio receiver of the invention in a form factor of a low power hand-held radio receiver is described as follows. The present invention provides an antenna assembly which is small enough to be integrated into a hand-held radio receiver of the present invention to provide the required selectable omnidirectional or directional reception of radio transmissions from mobile objects wearing the miniaturized transmitter of the present invention having a size approximately that of a pager. Omnidirectional reception permits the reception of transmissions from the mobile transmitters of the invention to determine whether each transmitter is inside or outside of the range set by the operator of the receiver. The directional reception has a front to back ratio of at least 10 db. to permit the user of the receiver to visibly determine, by viewing the lighted dots of the direction finding display, a direction of the transmitter relative to the receiver with an approximate resolution (beam width) producing maximum signal response (sensitivity) from 30xc2x0 to as little as 10xc2x0 in, for example, high frequency bands such as 902-928 MHz.
The antenna assembly implements the omnidirectional antenna function with a combination of an electrically conductive reflector and an electrically conductive loop. One end of the loop is always coupled to ground and the other end of the loop is coupled to an input of an RF amplifier. The electrically conductive reflector defines a cavity having a bottom, an opening, a surface extending from the bottom to the opening and an electrical output. The loop is positioned between the bottom and the opening of the cavity. The electrically conductive reflector and loop are electrically coupled together by a RF switch and to an input of the RF amplifier during operation as an omnidirectional antenna. The electrical coupling of the reflector is produced by the RF switch closing a conductive path to the loop and further a relatively close spacing between the loop and the reflector which produces mutual coupling via parasitic capacitance. The collective electrical coupling produced by the conductive coupling through the RF switch and the capacitive coupling improves the omnidirectional sensitivity over that achievable with the loop alone.
The antenna assembly implements the directional antenna function with the RF switch electrically coupling the reflector to ground which cuts off side lobes of reception of the loop to provide a highly focused beam width representing a maximum directional antenna response (sensitivity) of between 10xc2x0 and 30xc2x0. This is an optimal beam width for directional tracking of mobile objects, such as children and pets, with the radio receiver of the present invention.
When the antenna assembly is operated between 902 and 928 MHz. with a loop length of approximately 0.4 wavelength, the feed impedance appears to increase to a 20-40 ohm region which further facilitates impedance matching to a standard 50 ohm input impedance of a RF amplifier available in integrated circuit form. Moreover, the overall dimensions of the antenna assembly permit integration into a miniaturized receiver as described herein to monitor the transmitter relative to the set range and direction of the transmitter relative to the receiver to monitor the whereabouts of the objects which may be mobile, such as children, pets, etc.
An antenna assembly providing selectable omnidirectional or directional reception of radio transmissions in a frequency band in accordance with the invention includes an electrically conductive reflector defining a cavity having a bottom, an opening, a surface extending from the bottom to the opening and an electrical output; an electrically conductive loop electrically coupled to ground and having an electrical output for coupling to an RF amplifier, the loop being positioned between the bottom and the opening of the cavity; an RF switch having an input and first and second outputs, the RF switch having a first switching state electrically connecting the input to the first output and a second switching state electrically connecting the input to the second output, the first switch output being electrically coupled to ground and the second switch output being electrically coupled to the output of the electrically conductive loop; and wherein the first switching state provides the directional reception and the second switching state provides the omnidirectional reception of the radio transmissions. A closest separation of the loop from the cavity is preferably not greater than one-tenth of a wavelength of the frequency band of the radio transmissions and more preferably, a closest separation of the loop from the cavity ranges between 0.005 and 0.02 of a wavelength of the frequency band of the radio transmissions. The directional antenna has a beam width producing a maximum signal response to the radio transmissions between 10xc2x0 and 30xc2x0. The electrically conductive loop has a length between the output and the electrical coupling to ground greater than 0.2 of a wavelength of the frequency band and preferably approximately 0.4 of a wavelength of the frequency band; the cavity comprises at least two parts divided by a fold line which is the bottom of the cavity with the fold line being parallel to a directional axis of the directional reception and being a vertex of an oblique angle with first and second parts of the at least two parts of the cavity being sides of the oblique angle with ends of the first and second parts spaced from the vertex defining the opening of the cavity. Furthermore, the invention includes a circuit board having the RF amplifier mounted thereon with the electrical output of the electrically conductive loop coupled to the RF amplifier with the circuit board being located further from the electrically conductive loop than the vertex.
A preferred frequency band of operation of the antenna is between 902 to 928 MHz; a closest separation of the loop from the cavity preferably ranges between 0.1 and 0.2 inches; the loop is rectangular in cross section when viewed into the opening; and the directional antenna has a beam width producing a maximum signal reception of the radio transmissions between 10xc2x0 and 30xc2x0. The reflector, when viewed from the opening, is rectangular in cross section and has a width and a length across the width and length of the cross section which are less than 2.5 inches; and the loop, when viewed from the opening, is rectangular (preferably square) in cross section and has a width and a length across the width and length of the cross section which are less than 2 inches; and the loop is centered symmetrically with respect to the bottom of the reflector.
The antenna assembly further comprises a radio receiver containing the antenna assembly, the RF amplifier, a digital signal processor, coupled to the RF amplifier and a display, coupled to the digital signal processor, for displaying a received signal strength indicator. The digital signal processor controls switching of the first and second switching states and is responsive to a command to operate the antenna assembly as either an omnidirectional antenna during which the digital signal processor compares the received signal strength indicator representing the radio transmissions to a numerical value representing a set range between a radio transmitter broadcasting the radio transmissions and the radio receiver and generates an alert when-the comparison reveals that the radio transmissions are received from the radio transmitter outside the set range or a directional antenna during which the digital signal processor drives the display to display a relative magnitude of the received signal strength indicator so that the display of the magnitude of the received signal strength indicator is usable for direction finding of the transmitter relative to the radio receiver.